Resetting semi-passive stiffness damper and methods of use thereof

ABSTRACT

A resetting semi-passive stiffness damper (RSPSD) for use in damping movement. Embodiments of such a RSPSD include, for example, a pistoned cylinder connected to a grooved rack, and a spring-loaded lever arranged between the rack and a slotted channel above the rack. One end of the lever is allowed to move pivotably and vertically within the channel as the other end moves with the rack. A sensor is provided and communicates with a bypass valve on the cylinder. A change in the direction of movement of the rack forces the lever further into the slotted channel where it eventually triggers the sensor, which then sends a signal to open the bypass valve. As the rack continues to move, the lever eventually reverses direction, and is forced back down the slotted channel by a return spring. Upon leaving the sensing range of the sensor, the valve is signaled to close.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under grant No. 1235373awarded by the National Science Foundation. The government has certainrights in the invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/737,211 filed on Dec. 14, 2012.

TECHNICAL FIELD

The present invention is directed to stiffness dampers, systemscomprised of such stiffness dampers, and methods of use thereof.

BACKGROUND

Stiffness dampers of various design and size are known. Stiffnessdampers relevant to the invention are generally designed and used todamp movement. One such type of movement is the movement of structuresduring an earthquake.

Throughout history earthquakes have had a devastating impact on society,often resulting in significant economic losses and loss of life.According to the CATDAT Damaging Earthquakes Database, earthquakescaused global economic losses exceeding $500 billion as well as anestimated 20,000 fatalities. The Federal Emergency Management Agency(FEMA) estimates that the minimum average cost of earthquakes to theUnited States is $5 billion per year. However, a single large earthquakemay cost far more than the average annual estimate. For example, the1994 Northridge, Calif. earthquake alone caused as much as $26 billion,and it is predicted that another large earthquake along the San AndreasFault in southern California could result in 1,800 fatalities and morethan $200 billion in losses.

One way to reduce losses caused by earthquakes is to minimize thevulnerability of civil infrastructure. This can be achieved throughinfrastructure strengthening and/or the implementation of structuralcontrol.

Structural control is of interest here. Structural control can bebroadly characterized as active, passive, or semi-active, depending onthe hardware requirements. A significant amount of research has beenconducted in each category, and widespread application of structuralcontrol devices has been achieved. Of the three control types,semi-active control has recently received increased attention due to itsadaptability, minimal power requirement, and inherent stability. As aresult, several new and innovative semi-active control devices haveemerged. One of these, the resetting semi-active stiffness damper(RSASD), has proven effective in reducing the response of structures inthe presence of near-field ground motions. This is particularlyimportant, as this type of ground motion is characterized by high peakacceleration and high velocity pulse with long period, and isresponsible for the destruction and severe damage to civilinfrastructure.

An RSASD generally consists of a piston, a double-acting cylinder filledwith compressed air or hydraulic fluid, and a bypass loop with a valve(see FIG. 1a ). The cylinder is divided into two chambers by the pistonhead, and the chambers are connected by the bypass loop. When the bypassvalve is closed, the fluid in the cylinder is compressed due to theaction of the piston. When the valve is opened, energy stored in thefluid as a result of compression is turned into heat and dissipated.Therefore, for an RSASD installed in a structure, as shown in FIG. 1b ,the RSASD adds stiffness to the structure when the valve is closed, andremoves stiffness from the structure when the valve is opened.

The ability of a RSASD to add and remove stiffness from a structurecorrelates to an ability to store and then dissipate mechanical energy.Therefore, the RSASD is capable of extracting mechanical energy from astructure by opening and closing the valve at appropriate time instants.From this basic concept, a resetting mode concept emerged with the aimto maximize the amount of mechanical energy that is dissipated by aRSASD during a given cycle of motion. In the resetting mode, the valveremains closed until drift velocity equals zero, at which time the valveis pulsed open and closed, effectively resetting the stiffness of thedevice. As a result, a RSASD is always storing mechanical energy fromthe structure to which it is connected, and only dissipates energy whena maximum amount of energy storage has been reached.

Implementation of the resetting mode concept described above requiresthe use of feedback components such as encoders for determining pistonposition, a microcontroller for detecting a change in direction ofpiston movement, an electric servo-valve for regulating fluid flow, anda small power source for operating these components. One advantage ofthis resetting mode concept is that it may be implemented based on localinformation about each RSASD piston position, and does not requireknowledge of the structure response at other locations (i.e., it isdecentralized control logic).

Another advantage is that the control logic is response dependent, andtherefore does not need to rely on accurate information about structuralproperties which may be estimated incorrectly or change over time. Yetanother advantage of the RSASD is the displacement dependent nature ofthe control force delivered to the structure thereby. This isparticularly important for structures subject to near-field earthquakescharacterized by high velocity pulses where forces fromvelocity-dependent devices can often exceed control device capacity, mayrequire excessively large bracing systems for the devices, and canadversely affect the response of the structure. Displacement-dependentcontrol devices such as RSASDs are not susceptible to these effects, andare therefore well-suited for controlling the response of structuressubject to near-field motions.

In addition to the aforementioned advantages, structural control systemsusing RSASDs are simple, reliable, and relatively inexpensive relativeto other semi-active control systems. This can be attributed to theconstruction of the device, which is based on minor externalmodifications to existing pneumatic or hydraulic damper technology thatis well-developed and readily available.

Typical of semi-active control technologies, a RSASD also has severalcomplexities associated with its operation. First, the control law for aRSASD requires that stiffness be removed from an associated structurewhen it has reached maximum displacement, or zero velocity. This isachieved through a feedback control system consisting of a sensor,microcontroller, and a small actuator to control the valve. As a result,the feedback control system is disproportionately complex relative tothe feedback law.

Furthermore, the feedback control system is designed such that the valveis pulsed open and closed when the piston has reached its maximumdisplacement, i.e., when there is a change in sign of the pistonvelocity. However, this means that any noise (interference) in thesensor signal, or any high frequency small amplitude structuralvibrations, could also trigger the valve, thereby resetting the deviceat the wrong time. To prevent this, a deadband and a threshold on theposition signal must be used. The threshold is used to ensure that apredetermined minimum piston displacement has occurred before resettingthe device. The deadband eliminates resetting of the device based onlocalized peaks in the position signal that do not correspond with themaximum position of the piston.

As a result of the threshold, resetting only occurs after the piston hasmoved some minimum distance. As a result of the deadband, the valve istriggered a short time after the actual maximum displacement of thepiston has occurred.

It can be understood from the foregoing commentary that, while RSASDshave advantages when used for structural control, there is nonetheless aneed for a simpler device that provides similar results. Embodiments ofthe invention satisfy this need.

SUMMARY

Proposed herein is a less complicated stiffness damper device andsystems and methods of using such a damper device for structural controland possibly for other damping applications. More particularly, theinvention includes various embodiments of a resetting semi-passivestiffness damper (RSPSD), which is an innovative yet simple mechanismthat can replace the feedback system in the RSASD device while achievingthe same or a similar control effect.

The issue of noise in the RSASD feedback control system sensor iseliminated in the case of a RSPSD. Threshold and deadband are inherentin the RSPSD design, but may be controlled to eliminate unwantedresetting due to low level structural vibrations and local peaks. As aRSPSD does not rely on a feedback system, a RSPSD is more reliable andless expensive than a RSASD. A more reliable device at a reduce costthat provides the same control effect will translate to increasedacceptability in the structural controls community, and make thistechnology more attractive to structure owners.

An exemplary embodiment of a proposed RSPSD is schematically representedin FIG. 2. As shown, a piston is connected in series with a groovedrack. A spring-loaded triggering lever is arranged in a slotted channelabove the rack. The lever is allowed to move vertically and to rotateabout the end in the channel slot. The other end of the lever rests onthe grooves in the rack. A proximity sensor communicates with a bypassvalve on the cylinder.

Once the system is set in motion, the vertical position of the leverremains unaffected until the rack changes direction, at which time theend of the lever resting between the grooves on the rack is engaged.Upon engagement, further movement of the rack forces the lever to rotatewhile simultaneously driving it vertically in the slotted channel. Whenthe lever reaches a predetermined position, it triggers the proximitysensor, which then sends a signal to open the bypass valve. As the rackcontinues to move, the lever reaches its maximum vertical position,reverses direction, and is then forced back down the slotted channel bya return spring. Once the lever leaves the proximity of the sensor, thevalve closes. With the lever resting on the rack now oriented in theopposite position, the process is repeated when movement of the racknext changes direction.

It can be understood from the foregoing description of an exemplaryRSPSD that during operation thereof, the valve is pulsed open and closedeach time the piston changes direction, thereby permitting energy to beextracted from a vibrating structure. Furthermore, the only sensor usedis a proximity sensor that sends a set voltage to the bypass valve whilethe lever is within a certain range. Consequently, the complexity of theearlier described known RSASDs is eliminated.

In an alternate embodiment, a RSPSD could be modified to manually openand close the valve based on the position of the lever in the slottedchannel. In such an embodiment, all of the electrical componentsinherent to a typical RSASD could be removed, such that the resultingdevice would actually function as a resetting passive stiffness damper(RPSD). The RPSD would be a completely passive control technology.

BRIEF DESCRIPTION OF THE DRAWINGS

In addition to the features mentioned above, other aspects of thepresent invention will be readily apparent from the followingdescriptions of the drawings and exemplary embodiments, wherein likereference numerals across the several views refer to identical orequivalent features, and wherein:

FIG. 1a schematically represents the construction of a typical RSASD;

FIG. 1b depicts the RSASD of FIG. 1a installed in a structure;

FIG. 2 schematically represents an exemplary embodiment of a RSPSDaccording to the invention;

FIGS. 3a-3b schematically illustrate the position and movement of alever element of an exemplary RSPSD of the invention before and afterdisplacement of an associated rack element;

FIG. 4a graphically illustrates displacement of the lever element duringmovement of the rack element;

FIG. 4b graphically illustrates certain design parameters associatedwith the lever element;

FIGS. 5a-5b graphically represent the movement of a rack element andlever element, respectively, during testing of an exemplary RSPSD;

FIGS. 6a-6b are hysteresis charts illustrating the effects of a changein the sensing distance of a sensor element of an exemplary RSPSD;

FIG. 7 is a table comparing the hysteretic characteristics of anexemplary RSPSD to an exemplary RSASD;

FIGS. 8a-8b graphically depict the controlled vs. uncontrolleddisplacement and acceleration responses of an exemplary RSPSD during astructural control simulation;

FIGS. 9a-9b graphically depict the controlled vs. uncontrolleddisplacement and acceleration responses of an exemplary RSASD during astructural control simulation;

FIGS. 10a-10b graphically depict the energy dissipated by the exemplaryRSPSD and RSASD corresponding to the graphs of FIGS. 8a-8b and FIGS.9a-9b , respectively, when subjected to ground motion forces similar tothose seen during the Northridge earthquake;

FIG. 11 numerically presents the peak response data associated with theaforementioned exemplary RSPSD and RSASD during the simulation;

FIGS. 12a-12b schematically and respectively illustrate alternativeembodiments of amplified and non-amplified triggering lever assembliesthat may be used with a RSPSD of the invention;

FIGS. 13a-13b schematically and respectively illustrate furtheralternative embodiments of amplified and non-amplified triggering leverassemblies that may be used with a RSPSD of the invention;

FIG. 14 graphically compares the vertical displacement of the lever inthe channel versus the horizontal displacement of the damper piston forthe configurations shown in FIG. 2, FIG. 12a , and FIG. 13 a;

FIG. 15 depicts an exemplary scissor-jack mechanism that can beconnected to a lever of a RSPSD to provide more flexibility in thedesign thereof;

FIG. 16 is a plot of the relative horizontal displacement of between twojoints of the scissor-jack mechanism of FIG. 15 during use;

FIG. 17 schematically illustrates yet another exemplary embodiment of atriggering lever assembly that may be used with a RSPSD of theinvention, wherein a single mechanism acts as both the lever and theamplifying component;

FIG. 18 graphically illustrates the vertical displacement of a rodportion in an upper channel portion of the lever assembly of FIG. 17versus the horizontal displacement of a rack element for differentvalues of the ratio r, and

FIG. 19 schematically illustrates another alternative embodiment of atriggering lever assembly that may be used with a RSPSD of theinvention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENT(S)

Referring again to FIG. 2 and to FIG. 3a , it can be understood that anexemplary embodiment of a RSPSD 5 according to the invention willgenerally include a piston-containing cylinder 10 that is connected inseries with a grooved rack 15. A triggering lever 20, with a length (L),has one end arranged in a slotted channel 25 at a distance (y₀) abovethe rack 15. The channel may be associated with a vertical column 30,etc. The lever 20 is biased downward by a spring 35 located in thechannel 25.

The lever 20 is allowed to move vertically within the channel 25 and torotate about the end thereof that resides in the channel. The oppositeend of the lever 20 rests on the grooves in the rack 15, at a transversedistance (x₀) from the slotted channel 25 and at an initial orientationof (θ₀) with respect to the rack. A sensor 40, such as a proximitysensor, having a range (r) is located near the end of the channel 25farthest from the rack 15, and at a distance (h) above the end of thelever that resides in the channel. The lever 20 must travel a selecteddistance (s) to be within the sensing range of the proximity sensor 40.The proximity sensor 40 communicates with a bypass valve 45 on thecylinder 10.

Referring now to FIGS. 3a-3b , movement of the lever 20 and operation ofthe RSPSD 5 can be better understood. As shown in FIG. 3a , the rack 15is initially moving to the left. As long as the rack 15 continues tomove in the initial direction, the vertical position of the lever 20remains unaffected. However, as illustrated in FIG. 3b , when thedirection of movement of the rack 15 reverses (as indicated by thearrow), the rack-contacting end of the lever 20 becomes engaged with thegrooves of the rack 15. Upon engagement, further movement of the rack 15in the new direction forces the lever 20 to rotate while simultaneouslydriving the lever into the slotted channel 25 by some associateddistance (y(t)). Vertical movement of the lever also causes a transversemovement of the rack-contacting end thereof, the distance of movementbeing indicated by (x(t)). The total vertical distance the lever 20travels is denoted by (d). Using trigonometry, the vertical distance(y(t)) the lever travels as a function of the movement (x(t)) of therack can be expressed as:y(t)=√{square root over (L ² −[x ₀ −x(t)]²)}−(L−D)  (1)where (y(t)) is valid for 0<x(t)<2·(x₀).

When the lever 20 reaches a predetermined position, it triggers theproximity sensor 40, which then sends a signal to open the bypass valve45. As the rack 15 continues to move, the lever 20 reaches its maximumvertical position, reverses direction, and is then forced by the returnspring 35 back along the channel 25 in a reverse direction. Once thelever 20 leaves the sensing range (r) of the proximity sensor 40, thevalve 45 closes. With the rack-contacting end of the lever 20 noworiented in the opposite direction on the rack 15, the above-describedmovement of the lever and the associated process is repeated whenmovement of the rack next changes direction.

A plot of (y_(t)) for 0<x(t)<2·(x₀) is provided in FIG. 4a . Included onthe y-axis of the plot are the distance (s) the lever 20 must travel tobe within the range (r) of the proximity sensor 40, and the total traveldistance (d). For y(t)<s, the lever 20 is engaged but is not within therange of the sensor 40, and the valve 45 on the RSPSD will remainclosed. Once (s)≦y(t)<(d), an electrical signal is sent to the valve 45and it opens, the pressure in the RSPSD equalizes, and the damper forcedrops to zero. The valve 45 remains open as the lever 20 reaches (d),changes direction, and begins to move back down the channel 25. Once thelever 20 has moved out of range (r) of the sensor 40 y(t)<s, the voltagedrops to zero and the valve 45 closes again. Thus, pressure equalizationin the device takes place while 0<x(t)<2·x₀ and (s)≦y(t)<(d), acondition which was used to model resetting of the device 5 in variousnumerical simulations.

It is desirable that RSPSDs according of the invention perform similarlyto RSASDs, but using less feedback control components so as to simplifydevice operation and increase reliability. An ideal RSPSD would be ableto achieve instantaneous resetting of the control force with each changein direction of the rack. In other words, the valve would open, theforce would drop to zero, and the valve would close again at the instantthe piston velocity is zero. This operation would guarantee maximumenergy dissipation by the RSPSD. Comparable performance is notachievable using a RSASD due to delays in the signal, control logic, andvalve operation. In a RSPSD, this is not possible primarily due tolimitations of the mechanical components. That is, there is a delayassociated with the time it takes the lever to engage and begin movingvertically toward the sensor. This delay is represented by the bottomleft hand portion of the curve in FIG. 4a . During this time, the pistonhas already changed direction, and energy stored in the device ismomentarily being transferred back to the structure in which it isinstalled.

There is a second delay associated with a RSPSD that also prohibits itfrom achieving ideal performance. More specifically, once the valve isopen and the pressure equalizes, the valve will remain open until thelever leaves the sensing range of the proximity sensor, i.e., y(t)<s.This is represented by the top portion of the curve in FIG. 4a . Duringthis time, the stiffness in the device is zero and energy from thestructure isn't being stored. The combined time it takes the lever toreach the sensor and then leave the sensor will hereafter be referred toas the “resetting delay” of the device. Delays due to signal travel timeand valve opening/closing are incorporated in the top portion of thecurve in FIG. 4 a.

In order for a RSPSD to perform similar to a RSASD, both of the RSPSDdelays described above should be minimized through the design of therack-lever mechanism. This can be achieved by reducing the correspondingportions of the curve in FIG. 4a , that is, by reducing (d), (x₀), and(s-d). The sensing distance (s) is determined by the location (h) andrange (r) of the proximity sensor, while the distances (d) and (x₀)depend on the length (L) and initial orientation (θ₀) of the lever. Asproximity sensors come in a variety of standard ranges (e.g., 2 mm-12mm), and the height of the sensor above the lever is easily controlled,the primary design considerations are the lever length (L) and initialorientation (θ₀). FIG. 4b shows the change in (d) and (x₀) for30°<(θ₀)<60° and (L)=25.4 mm (1 in), and it is apparent that both designparameters decrease with increasing (θ₀).

Further decreases in (d) and (x₀) can be achieved by reducing the lengthof the lever (L). Therefore, the rack-lever mechanism can be design toreduce the resetting delay through the selection of design parameters(L), (θ₀), (h), and (r). However, consideration must also be given tothe practical limits on the minimum size of the rack-lever mechanismcomponents and the minimum time required for pressure equalization ofthe device. Previous research has shown the time for pressureequalization to be in the range of approximately 20-40 ms for pneumaticvalves.

Simulation

In order to demonstrate the feasibility of the rack-lever mechanism andto validate Equation (1), simulations were conducted in Solid Edge, acommercial software package that combines CAD and finite elementanalysis to aid in the design of complex systems. The rack-lever modelconsisted of a lever with a length (L)=70 mm (˜2.75 in) and (θ₀)=41°,resulting in a value of (d)=24.24 mm (˜1 in) and (x₀)=53 mm (˜2 in). Therack was driven through a sinusoidal displacement having anamplitude±150 mm (±6 in) and a frequency of 0.25 Hz, as is shown in FIG.5a . The vertical displacement of the lever was recorded for analysis.The same system was simulated in Matlab using Equation (1), and theresults are presented along with those from Solid Edge in FIG. 5 b.

FIGS. 5a-5b demonstrate that each time the rack changes direction(reaches its maximum positive or negative displacement) the lever isengaged and driven vertically to its maximum displacement. The extra“hump” at the beginning of the Solid Edge curve is due to a combinationof the initial orientation of the lever and motion of the rack,resulting in an immediate engagement and vertical (with respect to thedrawings herein) displacement of the lever. For the Matlab simulation,it was assumed that the rack moved so that the lever wasn't engageduntil the first change in rack direction. In practice, this initial‘hump’ would cause the RSPSD to reset at the start of the structuremotion. However, no energy is stored in the device at this time, and itis expected that this feature would have little effect on the overallRSPSD performance.

Another observation of FIG. 5b is the small amplitude oscillations ofthe lever present in the Solid Edge curve as the lever returns to thezero displacement position. These oscillations are attributed to theforce from the return spring acting on the lever (see FIG. 2), and dampout over a short period of time. Although the amplitude of thesevibrations is small, they could potentially cause unwanted resetting ofthe device. This can be avoided by designing the sensing distance (s) tobe greater than the maximum amplitude of the oscillation, orincorporating a small amount of additional damping in the system.Despite small differences between the two curves, FIG. 5b demonstratesthat Equation (1) accurately models the general motion of the rack-leversystem.

Hysteresis

Resettable stiffness devices work by storing and dissipating energy froma vibrating structure. The maximum energy dissipated in one cycle ofmotion is achieved by resetting the device at the instant the pistonvelocity equals zero—although as previously mentioned, this idealperformance is not achievable in practice due to delays in the resettingoperation.

The energy dissipation capacity of a resettable device is characterizedby the area under its hysteresis curve. To demonstrate the capacity ofthe RSPSD, simulations were conducted in Matlab based on the setup shownin FIG. 2. In order to reduce the resetting delay while consideringpractical limitations on the minimum dimensions of the mechanismcomponents, a lever length of (L)=12.7 mm (0.5 in) with initialorientation (θ₀)=30° was used. The resulting values of (x₀) and (d) were11 mm (˜0.5 in) and 6.35 mm (0.25 in) respectively. The height and rangeof the proximity sensor were varied to yield (s)=2 mm and (s)=6 mm, inorder to demonstrate the effect of (s) on the RSPSD performance. Theeffective stiffness of the pneumatic damper is assumed to be k_(d)=32.89kN/mm. For comparison, a hysteresis curve for an ideal RSASD was alsocomputed. For all simulations, the piston of the device was driventhrough a sinusoidal displacement having an amplitude of ±50.8 mm (±2in) and periods of 0.6 s, 1.8 s, and 5.4 s. The hysteresis curves forthe RSPSD for T=0.6 s and the two different values of (s) are presentedin FIGS. 6a-6b , along with the curve for the ideal RSASD.

A comparison of FIG. 6a and FIG. 6b reveals the effect of the sensingdistance (s) on the hysteresis of the device for a given value of (d).For a large sensing distance (s), there is a greater delay in thezeroing of the damper force once the piston has change direction. Thisis due to the lever traveling a longer distance prior to being sensedand opening of the valve takes place. As mentioned previously, thiswould correlate to stored energy being transferred back to thestructure, thereby reducing the area under the hysteresis curve comparedwith the ideal RSASD.

However, a large sensing distance (s) also means that the valve closesquicker due to the reduced time that the lever remains in the proximityof the sensor ((s)≦y(t)<(d)). As a result, energy begins being storedsooner after the force has zeroed. For a small sensing distance (s), theforce drops to zero sooner, and less energy is transferred back to thestructure. On the other hand, the lever spends more time in theproximity of the sensor, thereby increasing the time the valve is openand delaying the storage of energy after the force has zeroed. As aresult, the amount of energy dissipated by the device is again less thanthat with the ideal RSASD. In the end, the sensing distance (s) onlyshifts the hysteresis curve of the RSPSD, the same amount of energy isdissipated for (s)=2 mm and (s)=6 mm, and the energy is always less thanthat of the RSASD. This is shown in columns 3-4 of Table 1 (FIG. 7),which represents the energies dissipated (E) for both RSPSDs and theideal RSASD for different input periods.

The results indicate that the sensing distance (s) could be arbitrarilyset, as it doesn't affect the energy dissipative characteristics of thedevice. However, there is still a constraint on the minimum time thelever must remain in the proximity of the sensor so that the pressurehas time to equalize during opening of the valve, and this time isdirectly related to the sensing distance (s) and the total traveldistance (d). Previous research has determined this time to be in therange of 20 ms-40 ms depending on the characteristics of the valve.

In order to determine if the RSPSD meets this minimum requirement, thetime (t_(s)) that the lever is in the proximity of the sensor wascalculated for each RSPSD and all three input periods. The results ofthese calculations are presented in Columns 5 and 6 of Table 1, and showthat (t_(s)) is less for smaller periods and larger sensing distances(s). Furthermore, it is observed from Column 6 that (t_(s)) for the casewith T=0.6 s and (s)=6 mm is only 17 ms, and therefore does not meet theminimum requirement of 20 ms-40 ms. Based on the data presented in Table1, careful consideration should be given to the design of a RSPSD toensure that sufficient time is allowed for pressure equalization duringresetting while simultaneously minimizing the time the valve is open.

Structural Response Mitigation

In order to evaluate the potential of a RSPSD as a structural controldevice, additional simulations were conducted using asingle-degree-of-freedom (SDOF) building adopted from the relevantliterature (i.e., Lu and Lin, 2009). The building has a period of T=0.6s and a damping ratio of 5%. However, to investigate the performance ofthe RSPSD for SDOF buildings with larger periods, the stiffness of thebuilding was reduced to yield two additional buildings with periods ofT=1.8 sand T=5.4 s. The buildings were subjected to ground motionexperienced during the Northridge earthquake, with a peak groundacceleration of 0.84 g. For the RSPSD, the same design parametersdescribed in the previous section were used again, including sensingdistance values of (s)=2 mm and (s)=6 mm for the sensing distance. Forcomparison, the uncontrolled response of the building and the responseof the building with an ideal RSASD were also obtained.

FIGS. 8a-8b and 9a-9b graphically present the time history displacementand the absolute acceleration responses for the building with T=0.6 sfor the RSPSD and RSASD devices. For the RSPSD, the sensing distance (s)was 2 mm. The solid line in each graph represents the controlledresponse. The graphs indicate that both devices are effective inreducing the displacement response of the building, but less effectivein reducing the acceleration response.

FIGS. 10a-10b graphically represent the energy that would be dissipatedby each device during the earthquake, and demonstrate that the energydissipation capacity of a RSPSD is comparable to that of a RSASD.

The responses for the two remaining buildings, and for the RSPSD withsensing distances of (s)=2 mm and (s)=6 mm, are presented in Table 2(FIG. 11). Columns 2-5 of Table 2 show that the RSPSD is effective inreducing the peak displacement response of all three buildings, and itsperformance is comparable with that of the RSASD. Meanwhile, columns 6-9show that the RSPSD with a sensing distance of (s)=2 mm and (s)=6 mm,along with the RSASD, increase the absolute acceleration of the buildingcompared to the uncontrolled case. Furthermore, the performance of theRSPSD is again comparable with that of the RSASD.

It is interesting to note that the displacement response of the RSPSDwith a sensing distance (s)=6 mm is slightly smaller than that obtainedusing (s)=2 mm for the buildings with periods of 1.8 s and 5.4 s.Meanwhile, the RSPSD using a sensing distance (s)=2 mm outperforms theRSPSD using a sensing distance (s)=6 mm with respect to reducing theacceleration response of all three buildings. These results indicatethat although the energy dissipated by the RSPSD with a sensing distance(s)=2 mm and (s)=6 mm was the same for the sinusoidal displacementinput, the sensing distance does have a small effect on the response ofa flexible structure subject to random excitation.

Finally, the time (t_(s)) for pressure equalization was monitored forthe RSPSD for both sensing distance (s) and all three buildings. It wasdetermined that for (s)=6 mm, the value of (t_(s)) was less than theminimum required for pressure equalization for all three buildings. Onthe other hand, the minimum value of (t_(s)) for all three buildingsusing (s)=2 mm was 42 ms, which is at the high end of the requiredrange.

Amplified and non-amplified alternate embodiments of triggering leverassemblies 50, 85 that may be used with a RSPSD of the invention areschematically illustrated in FIGS. 12a-12b , respectively. Such leverassemblies 50, 85 may be used with the exemplary RSPSD 5 of FIG. 2, orwith other alternate embodiments not specifically shown or describedherein.

The triggering lever assembly 50 of FIG. 12a is shown to once againinclude a lever 55 with a length (L) and a second end arranged in aslotted channel 60. The lever 55 is again biased downward by a spring 65located in the channel 60. In this embodiment, the lever 55 and channel60 are mounted to a disc 70 in a manner that permits the disc to rotatewhile the channel and lever remain in the vertical position shown. Aproximity sensor 75 having a range (r) is again located near the end ofthe channel 60 farthest from the disc 70, and at a distance (h) abovethe end of the lever 55 that resides in the channel.

The disc 70 has a flange (not indicated) around its circumference thatprovides a contact point for a first end of the lever 55, while thesecond end of the lever is located in the channel. Rotation of the disc70 causes the first end of the lever 55 to move along the circumferenceof the disc, from point A to point A′. During this time, the second endof the lever 55 moves up and down within the channel 60 between point Bto point B′. This movement of the lever 55 within the channel 60triggers the proximity sensor 75 located at D.

For the configuration shown in FIG. 12a , the disc 70 rides on or ismounted on the damper piston 80 of the RSPSD or an element that movestherewith. Consequently, movement of the damper piston 80 or associatedelement will cause the above-described rotation of the disc 70 andresulting movement of the lever 55.

A primary benefit of the configuration shown in FIG. 12a versus thatshown in FIG. 2 is that the entire assembly may be mounted on the damperpiston by attaching it directly to the damper cylinder, without the needfor the rack 15 and the vertical column 30 of FIG. 2. The maindifference between the lever configuration of FIG. 12a and the leverconfiguration shown in FIG. 2 is that in the configuration of FIG. 12athere is less vertical displacement of the lever in the channel for thesame horizontal displacement of the piston damper. Equation (2) belowrelates the vertical displacement of the lever in the channel to thehorizontal displacement of the damper piston for the configuration shownin FIG. 12a .

$\begin{matrix}{{y(t)} = {\left\lbrack {\sqrt{L^{2} - \left\lbrack {x_{o} - {x(t)}} \right\rbrack^{2}} - \left( {L - d} \right)} \right\rbrack - {\quad{{\left\lbrack {\sqrt{R_{L}^{2} - \left\lbrack {x_{o} - {x(t)}} \right\rbrack^{2}} - \sqrt{R_{L}^{2} - x_{o}^{2}}} \right\rbrack\mspace{20mu}{x(t)}} = {{x_{o} - {R_{L}{\sin\left( {\varphi_{o} - {\varphi(t)}} \right)}\mspace{20mu}{\varphi(t)}}} = {{\frac{x_{p}(t)}{R_{L}}{y(t)}} = {\left\lbrack {\sqrt{L^{2} - \left\lbrack {R_{L}{\sin\left( {\varphi_{o} - \frac{x_{p}(t)}{R_{L}}} \right)}} \right\rbrack^{2}} - \left( {L - d} \right)} \right\rbrack - {\quad\left\lbrack {\sqrt{R_{L}^{2} - \left\lbrack {R_{L}{\sin\left( {\varphi_{o} - \frac{x_{p}(t)}{R_{L}}} \right)}} \right\rbrack^{2}} - \sqrt{R_{L}^{2} - x_{o}^{2}}} \right\rbrack}}}}}}}} & (2)\end{matrix}$where L is the length of the lever, R_(L) is the radius of the disc, φ₀is the angle formed between the lever and a radial line extending fromthe first end of the lever, x_(p)(t) is the distance of movement of thepiston, d is the total vertical travel distance of the lever within theslotted channel, and x₀ is the distance between the slotted channel andthe contact point of the lever with the circumferential flange of thedisc.

FIG. 12b depicts a variation of the triggering lever assembly 50 of FIG.12a . The triggering lever assembly 85 of FIG. 12b employs all of thevarious elements of the triggering lever assembly 50 of FIG. 12a ,including the lever 55 with its second end arranged in a slotted channel60, the lever 55 being biased downward by the spring 65 located in thechannel, and the proximity sensor 75 located near the end of the channel60 farthest from the disc 70. The lever 55 and channel 60 are againmounted to the disc 70 in a manner that permits the disc to rotate whilethe channel and lever remain in the vertical position shown.

In the triggering lever assembly 85 of FIG. 12b , however, the disc 70to which the lever and channel are mounted is the larger of two discspresent in the assembly. More particularly, in this embodiment, thelarger disc 70 is fixed to one end of a rigid shaft 95, while a smallerdisc 90 is fixed to the other end of the rigid shaft 95, such that thelarger disc 70, the smaller disc 90, and the shaft 95 rotate with thesame angular displacement, and the smaller disc 90 rides on or ismounted on the damper piston 80 of the RSPSD or an element that movestherewith. Furthermore, the disc-shaft assembly is mounted in a mannerthat permits the larger disc 70, the smaller disc 90, and the shaft 95to rotate while the channel and lever remain substantially in thevertical position shown. Consequently, linear movement of the damperpiston 80 or associated element will rotate the smaller disc 90 whichwill, in turn, cause a rotation of the larger disc 70 and a resultingmovement of the lever 55 as described above with respect to theconfiguration of FIG. 12 a.

In the configuration of FIG. 12b , the horizontal displacement/velocityof the damper piston 80 associated with the smaller disc 90 is relatedto the circumferential displacement/velocity of point A on the largerdisc 70 through the ratio of the radii of the two discs. By making theradius of the larger disc 70 greater than the radius of the smaller disc90, the circumferential displacement/velocity of point A on the largerdisc 70 is magnified relative to the horizontal displacement/velocity ofthe damper piston 80 according to the ratio of the radii of the largerdisc 70 to the smaller disc 90.

A benefit of this configuration is more flexibility in the RSPSD design.For example, the radial ratio of the larger disc 70 to the smaller disc90 may be adjusted so that point A on the larger disc 70 moves through acircumferential velocity that is twice that of the horizontal velocityof the damper piston 80. As a result, the lever 55 will move from pointA to point A′ faster, the lever will spend less time in the sensingrange of the sensor 75, and the resetting time will be reduced relativeto the configuration shown in FIG. 12a for the same horizontaldisplacement of the damper piston 80.

The configuration in FIG. 12b may also be used to achieve the sameresetting time as the configuration of FIG. 12a , but for a longerlever. Equation (3) below relates the vertical displacement of the leverin the channel to the horizontal displacement of the damper piston forthe configuration shown in FIG. 12b .

$\begin{matrix}{{y(t)} = {\left\lbrack {\sqrt{L^{2} - \left\lbrack {x_{o} - {x(t)}} \right\rbrack^{2}} - \left( {L - d} \right)} \right\rbrack - {\quad{{\left\lbrack {\sqrt{R_{L}^{2} - \left\lbrack {x_{o} - {x(t)}} \right\rbrack^{2}} - \sqrt{R_{L}^{2} - x_{o}^{2}}} \right\rbrack\mspace{20mu}{x(t)}} = {{x_{o} - {R_{L}{\sin\left( {\varphi_{o} - {\varphi(t)}} \right)}\mspace{20mu}{\varphi(t)}}} = {{\frac{x_{p}(t)}{R_{P}}{y(t)}} = {\left\lbrack {\sqrt{L^{2} - \left\lbrack {R_{L}{\sin\left( {\varphi_{o} - \frac{x_{p}(t)}{R_{p}}} \right)}} \right\rbrack^{2}} - \left( {L - d} \right)} \right\rbrack - {\quad\left\lbrack {\sqrt{R_{L}^{2} - \left\lbrack {R_{L}{\sin\left( {\varphi_{o} - \frac{x_{p}(t)}{R_{p}}} \right)}} \right\rbrack^{2}} - \sqrt{R_{L}^{2} - x_{o}^{2}}} \right\rbrack}}}}}}}} & (3)\end{matrix}$where L is the length of the lever, R_(L) is the radius of the largerdisc, φ₀ is the angle formed between the lever and a radial line of thefirst disc that extends to the first end of the lever, x_(p)(t) is thedistance of movement of the piston, R_(p) is the radius of the smallerdisc, d is the total vertical travel distance of the lever within theslotted channel, and x₀ is the distance between the slotted channel andthe contact point of the lever with the circumferential flange of thefirst disc.

Further alternate embodiments of amplified and non-amplified triggeringlever assemblies 100, 140 that may be used with a RSPSD of the inventionare schematically illustrated in FIGS. 13a-13b , respectively. Suchlever assemblies 100, 140 may be used with the exemplary RSPSD 5 of FIG.2, or with other alternate embodiments not specifically shown ordescribed herein.

The lever assembly 100 of FIG. 13a is shown to again include a lever 105with a length (L) and a second end arranged in a slotted channel 110.The lever 105 is again biased downward by a spring 115 located in thechannel 110. In this embodiment, the lever 105 and channel 110 areassociated with a column 120 or similar element that is in turn mountedto a disc 125 by an axle or in some other manner that permits the discto rotate while the channel and lever remain in the vertical positionshown. A proximity sensor 130 having a range (r) is again located nearthe end of the channel 110 farthest from the disc 125, and at a distance(h) above the end of the lever 105 that resides in the channel.

In this embodiment, the channel 110 is located on such that the contactpoint between the lever and the disc 125 occurs along the periphery(circumference) of the disc or a flange attached thereto, while thesecond end of the lever remains in the channel 110. Rotation of the disc125 causes the first end of the lever 105 to move along thecircumference of the disc 125, from point A to point A′. During thistime, the second end of the lever 105 moves up and down within thechannel 110 between point B to point B′. This movement of the lever 105within the channel 110 triggers the proximity sensor 130 located at D.

For the configuration shown in FIG. 13a , the disc 125 rides on or ismounted on the damper piston 135 or an element that moves therewith.Consequently, movement of the damper piston 135 or associated elementwill cause the above-described rotation of the disc 125 and resultingmovement of the lever 105.

A primary benefit of the configuration shown in FIG. 13a versus theconfiguration shown in FIG. 2 is that the entire assembly may be mountedon the damper piston by attaching it directly to the damper cylinder,without the need for the rack 15 and the vertical column 30 of FIG. 2.The main difference between the lever configuration of FIG. 13a and thelever configuration shown in FIG. 2 is that in the configuration of FIG.13a there is more vertical displacement of the lever in the channel forthe same horizontal displacement of the piston damper. Equation (4)relates the vertical displacement of the lever in the channel to thehorizontal displacement of the damper piston.

$\begin{matrix}{{y(t)} = {\left\lbrack {\sqrt{L^{2} - \left\lbrack {x_{o} - {x(t)}} \right\rbrack^{2}} - \left( {L - d} \right)} \right\rbrack + {\quad{{\left\lbrack {\sqrt{R_{L}^{2} - \left\lbrack {x_{o} - {x(t)}} \right\rbrack^{2}} - \sqrt{R_{L}^{2} - x_{o}^{2}}} \right\rbrack\mspace{20mu}{x(t)}} = {{x_{o} - {R_{L}{\sin\left( {\varphi_{o} - {\varphi(t)}} \right)}\mspace{20mu}{\varphi(t)}}} = {{\frac{x_{p}(t)}{R_{L}}{y(t)}} = {\left\lbrack {\sqrt{L^{2} - \left\lbrack {R_{L}{\sin\left( {\varphi_{o} - \frac{x_{p}(t)}{R_{L}}} \right)}} \right\rbrack^{2}} - \left( {L - d} \right)} \right\rbrack + {\quad\left\lbrack {\sqrt{R_{L}^{2} - \left\lbrack {R_{L}{\sin\left( {\varphi_{o} - \frac{x_{p}(t)}{R_{L}}} \right)}} \right\rbrack^{2}} - \sqrt{R_{L}^{2} - x_{o}^{2}}} \right\rbrack}}}}}}}} & (4)\end{matrix}$where L is the length of the lever, R_(L) is the radius of the disc, φ₀is the angle formed between vertical and a radial line extending to thefirst end of the lever, x_(p)(t) is the distance of movement of thepiston, d is the total vertical travel distance of the lever within theslotted channel, and x₀ is the distance between the slotted channel andthe contact point of the lever with the peripheral surface of the disc.

FIG. 13b depicts a variation of the triggering lever assembly 100 ofFIG. 13a . The triggering lever assembly 140 of FIG. 12b employs all ofthe various elements of the triggering lever assembly 100 of FIG. 13a ,including the lever 105 with its second end arranged in a slottedchannel 110, the lever 105 being biased downward by the spring 115located in the channel 110, and the proximity sensor 130 located nearthe end of the channel 110 farthest from the disc 125. The lever 105 andchannel 110 are again mounted to the disc by a column 120 or similarelement that is in turn mounted to the disc 125 by an axle or in someother manner that permits the disc to rotate while the channel and leverremain in the vertical position shown.

In the triggering lever assembly 140 of FIG. 13b , however, the disc 125to which the lever and channel are mounted is the larger of two discspresent in the assembly. More particularly, in this embodiment, thelarger disc 125 is fixed to one end of a rigid shaft 150, while asmaller disc 145 is fixed to the other end of the rigid shaft 150, suchthat the larger disc 125, the smaller disc 145, and the rigid shaft 150rotate with the same angular displacement, and the smaller disc 145rides on or is mounted on the damper piston 135 of the RSPSD or anelement that moves therewith. Furthermore, the disc-shaft assembly ismounted in a manner that permits the larger disc 125, the smaller disc145, and the shaft 150 to rotate while the channel and lever remain inthe vertical position shown. Consequently, linear movement of the damperpiston 135 or associated element will rotate the smaller disc 145 whichwill, in turn, cause a rotation of the larger disc 125 and a resultingmovement of the lever 105 as described above with respect to theconfiguration of FIG. 13 a.

In the configuration of FIG. 13b , the horizontal displacement/velocityof the damper piston 135 associated with the smaller disc 145 is relatedto the circumferential displacement/velocity of point A on the largerdisc 125 through the ratio of the radii of the two discs. By making theradius of the larger disc 125 greater than the radius of the smallerdisc 145, the circumferential displacement/velocity of point A on thelarger disc 125 is magnified relative to the horizontaldisplacement/velocity of the damper piston 135 according to the ratio ofthe radii of the larger disc 125 to the smaller disc 145.

A benefit of this configuration is more flexibility in the RSPSD design.For example, the radial ratio of the larger disc 125 to the smaller disc145 may be adjusted so that point A on the larger disc 145 moves througha circumferential velocity that is twice that of the horizontal velocityof the damper piston 135. As a result, the lever 105 will move frompoint A to point A′ faster, the lever will spend less time in thesensing range of the sensor 130, and the resetting time will be reducedrelative to the configuration shown in FIG. 13a for the same horizontaldisplacement of the damper piston 80.

The configuration in FIG. 13b may also be used to achieve the sameresetting time as the configuration of FIG. 13a , but for a longerlever. Equation (5) below relates the vertical displacement of the leverin the channel to the horizontal displacement of the damper piston forthe configuration shown in FIG. 13b .

$\begin{matrix}{{y(t)} = {\left\lbrack {\sqrt{L^{2} - \left\lbrack {x_{o} - {x(t)}} \right\rbrack^{2}} - \left( {L - d} \right)} \right\rbrack + {\quad{{\left\lbrack {\sqrt{R_{L}^{2} - \left\lbrack {x_{o} - {x(t)}} \right\rbrack^{2}} - \sqrt{R_{L}^{2} - x_{o}^{2}}} \right\rbrack\mspace{20mu}{x(t)}} = {{x_{o} - {R_{L}{\sin\left( {\varphi_{o} - {\varphi(t)}} \right)}\mspace{20mu}{\varphi(t)}}} = {{\frac{x_{p}(t)}{R_{P}}{y(t)}} = {\left\lbrack {\sqrt{L^{2} - \left\lbrack {R_{L}{\sin\left( {\varphi_{o} - \frac{x_{p}(t)}{R_{P}}} \right)}} \right\rbrack^{2}} - \left( {L - d} \right)} \right\rbrack + {\quad\left\lbrack {\sqrt{R_{L}^{2} - \left\lbrack {R_{L}{\sin\left( {\varphi_{o} - \frac{x_{p}(t)}{R_{P}}} \right)}} \right\rbrack^{2}} - \sqrt{R_{L}^{2} - x_{o}^{2}}} \right\rbrack}}}}}}}} & (5)\end{matrix}$where L is the length of the lever, R_(L) is the radius of the largerdisc, φ₀ is the angle formed between vertical and a radial line of thefirst disc that extends to the first end of the lever, x_(p)(t) is thedistance of movement of the piston, R_(p) is the radius of the smallerdisc, d is the total vertical travel distance of the lever within theslotted channel, and x₀ is the distance between the slotted channel andthe contact point of the lever with the peripheral surface of the firstdisc.

A comparison of the vertical displacement of the lever in the channelversus the horizontal displacement of the damper piston for theconfigurations shown in FIG. 2, FIG. 12a , and FIG. 13a is graphicallyillustrated in FIG. 14. With respect to differentiating the threecurves, y represents the vertical displacement of the lever in FIG. 2,y_(in) represents the vertical displacement of the lever in FIG. 12a,and y _(out) represents the vertical displacement of the lever in FIG.13 a.

FIG. 14 demonstrates that the effect of mounting the lever-channel on adisc is to increase or decrease the vertical displacement of the leverin the channel relative to a configuration with no disc. For a proximitysensor with sensing distance (s), this either increases or decreases thetime that the lever is in the sensing range of the sensor, resulting inan increase or decrease in the resetting time (relative to aconfiguration with no disc).

An objective of the different RSPSD configurations is to provide formore control over the relationship between the vertical displacement ofthe lever in the channel and the horizontal displacement of the damperpiston, leading to more flexibility in the RSPSD design. With thisobjective in mind, each of the configurations in FIG. 2, FIG. 12a , andFIG. 13a can be further modified by incorporating a scissor-jackmechanism 200 such as that shown in FIG. 15.

This exemplary scissor-jack mechanism 200 consists of four members ofequal length l pinned together at their ends (joints) and initiallyoriented at an angle α_(o) with respect to the scissor-jack centerline.One end of the scissor-jack is attached to a stationary point D, whilethe other is attached to the end of the lever B located in the verticalchannel. Vertical displacement of the end of the lever in the channelfrom point B to point B′ causes horizontal displacement of joint C onthe scissor-jack from point C to point C′, thereby triggering theproximity sensor located at E.

By incorporating a properly designed scissor-jack, the horizontaldisplacement of joint C can either be increased or decreased relative tothe vertical displacement of the lever in the channel, thereby providingmore flexibility in the design of the RSPSD. For the case where thehorizontal displacement of joint C is increased relative to the verticaldisplacement of the lever, the result will be a decrease in the amountof time that the proximity sensor is engaged relative to the case withno scissor-jack, for the same sensing distance s. As a result, thescissor-jack can be used to reduce the resetting time of the RSPSD. Forthe case when the horizontal displacement of joint C is decreasedrelative to the vertical displacement of the lever, the opposite istrue. Equation (6) below relates the horizontal displacement of joint Con the scissor-jack to the vertical displacement of the lever in thechannel.

$\begin{matrix}{{u(t)} = {2\;{l\left\lbrack {{\sin\left( {\cos^{- 1}\left( \frac{{2\; l\;\cos\;\alpha_{o}} - {y(t)}}{2\; l} \right)} \right)} - {\sin\;\alpha_{o}}} \right\rbrack}}} & (6)\end{matrix}$

FIG. 16 is a plot of the horizontal displacement of joint C versus thetotal vertical lever displacement from point B to point D, as shown inFIG. 15. FIG. 16 demonstrates that for values of the angle α_(o) equalto 10 and 30 degrees, the horizontal displacement of joint C isinitially increased relative to the vertical displacement of the lever.However, as the vertical displacement of the lever increases, the amountof relative increase in the horizontal displacement of the leverdecreases. Meanwhile, for values of the angle α_(o) equal to 60 degrees,the horizontal displacement of joint C is always less than the verticaldisplacement of the lever.

Yet another exemplary embodiment of the invention is shown in FIG. 17.In this exemplary embodiment of a RSPSD 250, a single mechanism acts asthe lever and amplifying component of the system. In this configuration,the lever 255 is extended so that while one end is in contact with arack 270 at point A, the other end is pivotally connected to a rod CD atpoint C such that the lever ABC is free to rotate about rod CD at pointC. The extended lever is rotatably connected to a lower channel 260 atpoint B such that the lever is allowed to move vertically within thelower channel and to rotate about point B thereof that resides in thelower channel. The rod CD is connected to an upper channel 265 at pointD such that the rod is allowed to move vertically within the upperchannel and to rotate about the end D thereof that resides in the upperchannel.

Horizontal displacement of the rack 270 drives the end of the lever incontact with the rack from point A to point A′. During this time, leverABC moves up the lower channel from point B to point B′, whilesimultaneously rotating about point B which resides in the channel. Thevertical displacement and rotation of lever ABC is accompanied byvertical displacement and rotation of rod CD, thereby causing the end ofrod CD at point D in the upper channel to move from point D to point D′,and triggering the proximity sensor located at point E.

In this configuration, the horizontal displacement/velocity of the endof the lever in contact with the rack at A from point A to point A′ isrelated to the horizontal displacement/velocity of point C from point Cto point C′ by the ratio of the lever ABC arm lengths r(r=BC|AB). Bymaking the length of arm BC longer than that of arm AB, the horizontaldisplacement/velocity of point C is magnified relative to the horizontaldisplacement/velocity of point A according to r. The advantage of thisconfiguration is more flexibility in the RSPSD design. For example, theratio of the lever arm lengths r may be adjusted so that the horizontalvelocity of point C is twice that of the horizontal velocity of point A.If the horizontal displacement of point C is made equal to the rackdisplacement shown in FIG. 2, the rod will move from point C to point C′faster, the lever will spend less time within the sensing range of thesensor, and the resetting time will be reduced (relative to theconfiguration in shown in FIG. 2).

The configuration of FIG. 17 may also be used to achieve the sameresetting time as the configuration of FIG. 2, but for a longer lever.In this regard, equation (7) relates the vertical displacement of rod CDin the upper channel 265 to the horizontal displacement of the rack 270.y(t)=(1+r)[√{square root over (L ₁ ² −[x _(o) −x _(p)(t)]²)}−√{squareroot over (L ₁ ² −x _(o) ²)}]+[√{square root over (L ₂ ² −r ² [x _(o) −x_(p)(t)]²)}−√{square root over (L ₂ ² −r ² x _(o) ²)}]  (7)

FIG. 18 graphically illustrates the vertical displacement of the rod CDin the upper channel 265 versus the horizontal displacement of the rack270 for different values of the ratio r, and demonstrates the amplifyingeffect of the lever ABC in the configuration shown in FIG. 17. It shouldbe noted that the alternate configuration shown in FIG. 17 could also bemounted in the disc configurations shown in FIGS. 12a-12b and 13a -13 b.

Another alternate embodiment of a triggering lever assembly 300 that maybe used with a RSPSD of the invention is schematically illustrated inFIG. 19. Such a lever assembly 300 may be used with the exemplary RSPSD5 of FIG. 2, or with other alternate embodiments not specifically shownor described herein. Furthermore, it should be understood that thetriggering lever assemblies of FIGS. 3a-3b, 12a-12b, 13a-13b and 17 maybe modified to move the proximity sensor to a position like that shownin FIG. 19.

The triggering lever assembly 300 of FIG. 19 is shown to once againinclude a lever 310 with a length (L) and a first end arranged in aslotted channel 315 at a distance (y_(e)) above a rack 305. The lever310 is again biased downward by a spring 320 located in the channel 315.

As with previous embodiments, the lever 310 is allowed to movevertically within the channel 315 and to rotate about the end thereofthat resides in the channel. The opposite end of the lever 310 rests ongrooves in the rack 305, at a transverse distance (x₀) from the slottedchannel 315 and at an orientation of (θ) with respect to the rack. Asthe angle θ between the lever 310 and the rack 305 is time dependent,the angle is represented in FIG. 19 as θ(t).

In a manner similar to that of the embodiment shown in FIGS. 3a-3b ,when the rack 305 of the assembly 300 of FIG. 19 is moving to the left,the vertical position of the lever 310 remains unaffected. However, whenthe direction of movement of the rack 305 reverses and moves to theright (as indicated in FIG. 19), the rack-contacting end of the lever310 becomes engaged with the grooves of the rack 305 and, uponengagement, further movement of the rack in this direction forces thelever to rotate while simultaneously driving the lever into the slottedchannel 315 by some associated distance (y(t)). Horizontal movement ofthe rack 305 also causes a like movement of the rack-contacting end ofthe lever 310, the distance of movement at any given time beingindicated by (x_(r)(t)).

Like the previously described triggering lever assemblies, thistriggering lever assembly 300 also employs a proximity sensor 325 thatcommunicates with a bypass valve 45 on the cylinder 10. However, unlikethe previously described exemplary triggering lever assemblies, thisvariation of the triggering lever assembly 300 employs a proximitysensor 325 that is located outside of the channel 315. Moreparticularly, the proximity sensor 325 is located at a distance (h)below the end of the channel 315 that is closest to the rack 305, and isoriented such that the sensing direction is substantially perpendicularto the plane in which the rack-lever mechanism motion occurs (i.e., thesensing direction extends perpendicularly from the page in FIG. 19).

In the configuration of FIG. 19, a sensing material (not specificallyshown) that triggers the proximity sensor 325 may be mounted to the bodyof the lever 310 rather than the end that resides in the channel 315.Alternatively, the lever 310 itself may trigger the sensor 325. Thesensing material (when present) is also located at a height such that itpasses over the proximity sensor when the tip of the lever 310 isengaged by the rack 305 and moves through a distance 2(x_(o)). Thesensor 325 is triggered anytime the lever is within the sensing range(s) of the sensor. A benefit of this configuration is that the leverlength (L) and the initial lever position (x_(o)) can be designed sothat very small displacements of the rack 305 will trigger the proximitysensor 325 and reset the damper, thereby allowing for energy to bereleased and then quickly stored again each time the rack changesdirection.

While certain exemplary embodiments of the present invention aredescribed in detail above, the scope of the invention is not to beconsidered limited by such disclosure. Rather, modifications arepossible without departing from the spirit of the invention. Forexample, the grooved or toothed rack elements shown and described hereinmay be replaced with another type of actuator that simply includes alever contacting surface of sufficiently high friction to displace thelever as described. Another, non-limiting modification, may include theuse of a programmable valve that would remain open for a set amount oftime once opened, regardless of vibration characteristics. RSPSDembodiments according to the invention may also have uses outside thefield of structural control. For example, such other uses may include,without limitation, vehicle suspensions and aircraft landing gear.

What is claimed is:
 1. A resetting semi-passive stiffness damper(RSPSD), comprising: a reciprocatable piston located within a cylinder;a linearly displaceable grooved rack coupled at one end to the piston;an actuator coupled between a structure to be damped and an opposite endof the grooved rack; a triggering assembly, the triggering assemblycomprising: a displaceable, spring-loaded lever arranged between thegrooved rack and a slotted channel residing above the grooved rack, afirst end of the lever in contact with the grooved rack and a second endof the lever movably residing in the channel, and a sensor residing inthe channel and located and adapted to detect the lever when the secondend of the lever reaches a predetermined position within the channel,and to resultantly transmit an electrical valve open signal upondetection of the lever; and a bypass valve associated with the cylinderand in communication with the sensor of the triggering assembly, thebypass valve adapted to receive the electrical valve open signal fromthe sensor and to open in response thereto.
 2. The RSPSD of claim 1,wherein vertical travel of the lever in the channel is related to thetravel distance of the rack by a formula written as:y(t)=√{square root over (L ² −[x ₀ −x(t)]²)}−(L−d) where L is the lengthof the lever, x₀ is the distance between the slotted channel and acontact point of the lever with the rack, x(t) is the distance ofmovement of the rack and the first end of the lever, d is the totalvertical travel distance of the lever within the slotted channel, andy(t) is valid for 0<x(t)<2(x₀).
 3. The RSPSD of claim 2, wherein thebypass valve is closed while the lever position satisfies the formulay(t)<s, where s is the vertical distance the lever must travel withinthe slotted channel to be within the sensing range of the sensor.
 4. TheRSPSD of claim 2, wherein when the lever reaches a position wheres≦y(t)<d, the bypass valve will open, the pressure in the RSPSD willequalize, and the damper force will drop to zero.
 5. The RSPSD of claim4, wherein the bypass valve will remain open until the lever moves to aposition that satisfies the formula y(t)<s.
 6. The RSPSD of claim 2,wherein pressure equalization will take place while 0<x(t)<2(x₀) ands≦y(t)<d.
 7. The RSPSD of claim 1, further comprising a lever returnspring located within the slotted channel, the lever return springadapted to bias the lever toward the grooved rack.
 8. The RSPSD of claim1, wherein the sensor of the triggering assembly is further adapted totransmit an electrical valve close signal when the sensor does notdetect the lever.
 9. The RSPSD of claim 1, wherein the sensor is aproximity sensor.
 10. The RSPSD of claim 1, wherein: a back-and-forthmovement of the grooved rack will cause a reciprocating movement of thesecond end of the lever within the slotted channel; and wherein thereciprocating movement of the second end of the lever within the slottedchannel will cause the sensor of the triggering assembly to repeatedlyopen and close the bypass valve to accordingly transfer energy betweenthe structure to be damped and the RSPSD.
 11. A resetting semi-passivestiffness damper (RSPSD), comprising: a reciprocatable piston locatedwithin a cylinder; a linearly displaceable grooved rack coupled at oneend to the piston; an actuator coupled between a structure to be dampedand an opposite end of the grooved rack; a triggering assembly, thetriggering assembly comprising: a displaceable lever arranged betweenthe grooved rack and a slotted channel residing above the grooved rack,a first end of the lever in contact with the grooved rack and a secondend of the lever movably residing in the channel and in contact with areturn spring located therein, and a sensor residing in the channel andlocated and adapted to detect the lever when the second end of the leveris within the sensing range of the sensor, the sensor further adapted totransmit a valve open signal upon detection of the lever and to transmita valve close signal when the lever is outside the sensing range of thesensor; and a bypass valve associated with the cylinder and incommunication with the sensor of the triggering assembly so as toreceive the electrical open and close signals from the sensor and toopen and close in response thereto.
 12. The RSPSD of claim 11, whereinvertical travel of the lever in the channel is related to the traveldistance of the rack by a formula written as:y(t)=√{square root over (L ² −[x ₀ −x(t)]²)}−(L−d) where L is the lengthof the lever, x₀ is the distance between the slotted channel and acontact point of the lever with the rack, x(t) is the distance ofmovement of the rack and the first end of the lever, d is the totalvertical travel distance of the lever within the slotted channel, andy(t) is valid for 0<x(t)<2(x₀).
 13. The RSPSD of claim 12, wherein thebypass valve is closed while the lever position satisfies the formulay(t)<s, where s is the vertical distance the lever must travel withinthe slotted channel to be within the sensing range of the sensor. 14.The RSPSD of claim 12, wherein when the lever reaches a position wheres≦y(t)<d, the bypass valve will open, the pressure in the RSPSD willequalize, and the damper force will drop to zero.
 15. The RSPSD of claim14, wherein the bypass valve will remain open until the lever moves to aposition that satisfies the formula y(t)<s.
 16. The RSPSD of claim 12,wherein pressure equalization will take place while 0<x(t)<2(x₀) ands≦y(t)<d.
 17. The RSPSD of claim 11, wherein the sensor is a proximitysensor.
 18. The RSPSD of claim 11, wherein: a back-and-forth movement ofthe grooved rack will cause a reciprocating movement of the second endof the lever within the slotted channel; and wherein the reciprocatingmovement of the second end of the lever within the slotted channel willcause the sensor of the triggering assembly to repeatedly open and closethe bypass valve to accordingly transfer energy between the structure tobe damped and the RSPSD.